Radio-frequency plasma containing device



Feb. 20, 1962 A. J. ULRICH ETAL RADIO-FREQUENCY PLASMA CONTAINING DEVICE Filed Aug. 14. 1959 2 Sheets-Sheet 1 lie yend:

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flttorng .U fitid at The present invention relates generally tomethods and means for containing and compressing plasma discharges. More particularly, it relates, to the use of radio-frequency electromagnetic waves to contain and compress plasma discharges which may produce thermonuclear-derived neutrons and which may ultimately provide useful power.

It is of importance to define certain terms employed in the following description. The term plasma is herein taken to mean a space-charge neutralized ion cloud containing substantially equal numbers of positive ions and negative electrons. The term radio-frequency machine signifies for the purpose of the description a reactor in which confining force on the plasma is developed 'by periodic magnetic of electromagnetic fields.

. In addition to the well known fission process, there is another reaction which may be utilized for the release of neutrons and energy. This reaction is generally termed fusion in that it includes the combination of the nuclei ,of two light elements into a nucleus of asingle heavier element. Briefly, certain light elements such as deuterium, tritium, and lithium interact under proper circumstances to produce thermonuclear reactions. Such reactions result from the fusion combination of the nuclei of the reactant elements with the release of subatomic particles such as neutrons and protons, and energy in the 3 He (.8 mev.) n(2.5 mev.)

D D i i T (1.0 mev.) p(3.0 mev.) (2) D+T He +n+17.6 mev.

For a more detailed description of the above reactions reference is made to an article authored by Richard F. Post in the Reviews of Modern Physics, September 1956, relating to a survey of controlled fusion research and to vols. 31 and 32, Peaceful Uses of Atomic Energy, UN, 1958.

The neutrons released in the fusion reactions may be used in well known ways for the irradiation of elements and other substances. The resulting isotopes are useful in industry as well as in basic research. The attendant protons are also useful as an irradiating medium. Ref erence is made to vol. 19, Peaceful Uses of Atomic Energy, UN, 1958, for further information regarding industrial and research applications of isotopes.

The thermonuclear reaction wouldalso be highly valuable as a source of power. In order for the thermonuclear reaction to be feasible for production of power, however, there must be an energy balance in the direction of energy released from the reaction and to'this end, a minimization of losses in the reaction is necessary. In order to produce a desired energy balance it is necessary for the entire fuel charge or reactant material to be heated to a kinetic temperature sufficient to produce a substantial reaction rate by virtue of the collisions of the fuel nuclei. These kinetic temperatures correspond to mean particle energies of tens or even hundreds of thousands of electron volts. Concerning the particular reaction parameters, such as minimum temperatures and densities azzsss Patented Feb. 20, i962 necessary for producing thermonuclear reactions for the desired energy balance, reference is made to the book, Project Sherwood, The US. Program in Controlled Fusion, by Amasa S. Bishop, Addison Wesley Publishing Co. Inc., 1958.

It will be appreciated that thermonuclear power provides a possible source of almost unlimited energy and that the energy release in thermonuclear reactions is relatively spectacular as compared to chemical fuels, particularly when the cost of the fuel materials is taken into consideration. The relative abundance of the elements employed in thermonuclear reactions as fuel materials points to the eventual widespread use of this type of reaction. Although known chemical fuels, such as oil and coal, occur in the earths surface in limited quantities, there is an almost unlimited supply of thermonuclear fuel material to be found in the ocean. The methods and means set forth herein provide a manner of producing thermonuclear reactions which may ultimately lead to successful harnessing of the energy released therefrom.

It will be noted that the reactions, hereinbefore mentioned, involve the collision of atomic particles having like'electric charges. A high energy barrier exists between the two particleswhich must be overcome for a thermonuclear reaction to occur. It is known that thermonuclear reactions provide in large amounts the energies of the sun and the stars. In these bodies the reaction elements are maintained at a very high energy level and in a relatively dense state so that the probability of collision to produce a thermonuclear reaction is increased. The sun and stars utilize their very high gravitational fields to contain the reaction materials in the high density state. The gravitational field on earth, however, is not large enough, and, therefore, the forces of the sun and the stars must be artificially duplicated if thermonuclear reactions are to be produced.

it has been determined that thermonuclear reactions are most likely to occur in high density plasmas in which the kinetic energy of the particles making up the plasma are at a very high energy level. It is necessary that the plasma be contained at a requisite density and that the particles be excited to a proper temperature with respect to the density with either a continuous or some sort of pulsed mode of operation. Although various approaches are possible to the provision of circumstances to further the thermonuclear reaction, the particular approach disclosed herein employs radio-frequency electromagnetic fields wherein a plasma is contained and compressed to increase the density and temperature of the ions therein.

One of the major problems that has been apparent since the conception of the thermonuclear program is the confinement of a plasma of sufiiciently high density and temperature for a long enough period of time to enable a thermonuclear reaction to take place. The confinement problem has been limited for the most part to the use of steady magnetic fields, although electric fields were given some consideration early in the project. The major deterring factor in the use of electric fields was thatsuch a field exerts oppositely directed forces on the electrons and positive ions of the plasma so that, if the field was made to confine one component, the other would have a tendency to escape. This invention, however, describes methods and means for confining the plasma particles by electromagnetic fields regardless of their charge, causing them to be compressed in a high density configuration and maintained in a stable condition for a finite period of time.

It is an object of the present invention to provide a method and means for confining and compressing a plasma discharge.

It is another object of the present invention to pro- '9 a) vide a method and means for producing a stabilized plasma discharge.

It is another object of the present invention to provide a method and means for containing and compressing a plasma discharge by means of radio-frequency electromagnetic fields.

It is a further object of the present invention to provide a device using radio-frequency electro-magnetic fields which is useful as asource of neutrons and other irradiating particles.

Still another object of the present invention is to provide a radio-frequency device which may ultimately lead to the production of useful power by means of nuclear reactions. Various other objects and advantages will be apparent to those skilled in the art from the following description and accompanying drawings wherein:

FIGURE 1 is a graph showing the total resistive power input and the total power output plotted against the plasma energy release for representative values of the plasma density;

FIGURE 2 shows a TE electromagnetic field in a spherical cavity with a central conducting sphere;

FIGURE 3 shows a rotating field in a spherical cavity with a central conducting sphere;

FIGURE 4 shows a square wave perturbation in the inner face between the plasma and the confining electromagnetic field;

FIGURE 5 is a vertical section view of a device for the practice of this invention with some of its components shown schematically;

FIGURE 6 is an enlarged, fragmentary view of the portion encircled and referenced 6 in FIGURE 5 showing the details of an electromagnetic field coupling loop;

FIGURE 7 is an enlarged, fragmentary section view of the portion encircled and referenced 7 showing the de tails of construction of the vacuum manifold surrounding the housing. With the steady state magnetic fields such as employed in the well known pinch effect and other magnetic plasma confining machines, instabilities in the confining fields are apt to appear which permit the plasma to escape through the confining walls of the device with substantial loss of power. With a periodic confining force such as utilized in the radio-frequency plasma confining machines, there is opportunity to take advantage of dynamic stabilization effects. Dynamic stabilization of otherwise unstable systems does exist in nature as is evidenced by the frequently cited examples of the inverted pendulum and the alternating gradient focussing system proposed for high energy particle accelerators. It is also possible for a column to support a periodic compression load with an average value greater than the critical buckling force although in this situation, the stability zones are quite narrow.

In the case of radio-frequency confining fields, the organized motions resulting from instabilities tend to be oriented by the confining field at the plasma surface. If

this field is then caused to vary rapidly, particularly in direction, the instability motions lose coherence with the field structure and will not develop sufliciently to allow escape of the plasma.

To contain a plasma discharge in a stable configuration and compress it to a density and temperature at which a thermonuclear reaction could take place, a gas formed of light elements such as deuterium or tritium is introduced into a cavity in which an electromagnetic field, preferably rotatable around an axis, may be established. As the electromagnetic field forms and constricts the gas, the temperature increases and the gas becomes fully ionized. As the density of the gas increases, and the energy of the gas ions increases, the collision probability for the positive ions also increases. If the collision probability is sufficiently high a thermonuclear reaction involving the fusion of positive ions will occur, resulting in the release of high energy subatomic particles, such as neutrons and protons, and the release of high energy fusion products, such as helium.

An estimate of minimum power requirements for net power output can be made without reference to particular apparatus. Taking as representative a spherical plasma of diameter 0.2 meter, a graph of the total energy input is displayed in FIG. 1. In the figure, the resistive power input and the total power output are each plotted against the plasma energy release for representative values of the plasma density. The curves are representative of the D-T reaction, the small (approximately 1%) contribution from D-D reactions being ignored. The graph shows that the lower limit of density for a plasma of this particular size is in the neighborhood of IO /meter and that at least 0.1 to 1 megawatt of power must be involved.

Turning now to the containment of a plasma discharge, it is well known that a charged particle in a standing electromagnetic wave system experiences, in addition to the forces driving its oscillatory motion, a time average force in a predicted direction. If then,.a situation can be created in which the gradient of the time average electric field is everywhere directed toward a common central zone, it is possible to contain a plasma in this zone at least up to a certain density at which instability motions might ensue. The electromagnetic field employed in this invention and hereinafter described has this property. In actual operation of the radio frequency machine, hereinafter described, neutral gas atoms are ionized by collision with electrons in the region of high electric field,

the resulting ions and electrons being driven partly to- I ciple of radio-frequency confinement, it is instructive to begin with an estimate of probable reactive power requirements. Reactive power as used herein is defined as the peak field energy times the angular frequency of the confining field. An estimate of the reactive power may be made using the expression 2wnkT (field volume) where w is the angular frequency of the confining field, n is the density of the plasma, and kT is the energy of the plasma corresponding to temperature T. A plasma temperature of approximately 1 kev. will produce a substantial number of neutrons'at a plasma density of about 2 10 particles per meter The volume of the plasma will be approximately 4.19 10- meters the field volume being approximately l0 the volume of the plasma. Using a frequency of megacycles/second, the react1ve power is calculated to be about l.69 10 megawatts which is quite large and can only be obtained in a very high Q system, Q being a figure of merit equal to maximum energy stored energy lost per cycle at resonance. A comparatively long time scale is thereby implied since, in order to take advantage of the high Q, a time of the order of q/w must be allowed for the field energy to build upto near its final value. It is thus important to eliminate any contact between the plasma and material walls or electrodes due to the attendant large cooling effect.

To fulfill this requirement, the electromagnetic field must be such that no electric field lines connect the plasma with the metal walls. FIG. 2 shows an electromagnetic wave of the TE mode in a spherical cavity which has its electric field lines closing on themselves and thus meeting the requirements. As indicated in FIG. 2, the electromagnetic energy oscillates between a purely toroidal E field and poloidal B field. It will be noted, however, that there are two points, A and A at which the confining force is zero and such points will exist for any 18 to the evacuated cavity 14 through ceramic domes 22' and 22a. Reference is made to FIG. 6 which shows the construction of the ceramic dome 22 with the coupling loop 20 therein. Each dome 22 is fabricated of a strong ceramic such as pyroceram and is welded to a metal ring 23 attached to the outside of the steel sphere 12.

The ceramic domes 22 enclosing the coupling loops 20 for one mode of RF energy are positioned on a great circle of the sphere which is included in the plane of FIG. b The other mode of RF energy is generated by the four tetrode tubes (not shown) which have their coupling loops within the ceramic domes 22a which are positioned on a great circle lying within a plane through the axis of rotation BB and perpendicular to the plane of FIG. 5. When the two sets of tetrodes are synchronized to operate in time quadrature, a revolving electromagnetic field such as is shown in FIG. 3 will be established in the cavity 14.

The eight tube feed system as shown in FIG. 5 was chosen in preference to a simpler two tube feed system to keep the cavity Q as high as possible. RF power radiation through an aperture varies about as the 6th power of the diameter whereas the peak power trans mission is limited by voltage breakdown and hence varies only as the square of the diameter. The reduction in Q is therefore less for the eight tube system.

Table I Filament power ..kw 2.7 Plate voltage v-.. 24 Plate current (peak) amp 13 Power output (peak) kw 170 Power gain 100 Cooling water g.p.m 20

The beam tetrodes 18 are adapted to operate as selfexcited, grid-driven oscillators and are so connected that they follow any resonant frequency changes that occur during the formation and subsequent confinement of the plasma. The empty cavity resonates at 715 megacycles and a fully developed plasma core of /3 the radius of the cavity raises this to 800 megacycles. The RF generating system may be made to follow this frequency shift by minimizing the stored energy in the external circuit elements and by making the main cavity an integral part of the feed-back path for the oscillators. By connecting the grid 24 of each tube 18 through an amplifier and phase shift network 26 to a coupling loop 27 penetrating into the cavity 14, the frequency changes caused by the compression of the plasma will be fed back to the tube adjusting the frequency output accordingly.

The device as described may be used in a single pulse or a repetitive pulse operation. The output of the tubes 18 may be pulsed by pulsing means within the plate supply 28 furnishing power to each of the tubes 18. The device may also be operated as a steady state machine by eliminating pulsing means within the plate supply 28, providing RF tubes capable of megawatt operation on a steady basis are used.

As hereinbefore discussed, a steady magnetic field through the cavity 14 parallel to the axis of rotation B-B' will serve to aid in stabilizing the electron orbits over the plasma interface thereby reducing the probability of escape. To establish the magnetic field, a coil 29 is provided, and is oriented as shown in FIG. 5, so that the magnetic field lines set up by a current therethrough supplied by a current source 29a, will have a component parallel to the axis of rotation BB'. A field strength approximately equal to the magnetic field established by the RF system or 420 gauss will suflice for this purpose.

A high quality vacuum system to insure adequate control of gas purity is an essential of the facility. A critical feature of this system is the design of cavity pumping ports. These ports must have adequate evacuating conductance, but should not appreciably affect the cavity Q nor contribute to the excitation of spurious RF modes. A study of the current density vectors at the surface of a cavity for the electromagnetic excitation of the type used in this device will show that the vector locus at the great circle which is perpendicular to the axis of rotation B-B and the plane of FIG. 5 is a straight line, and that the RF current flow is always perpendicular to the plane. The average magnitude of the current density is a minimum at the plane. A large number of small slots 30 elongated in the direction of the current vector will thus provide a minimum of distortion of the electromagnetic field modes. In the device herein described, 60 slots, 6 millimeters wide and 3 centimeters high are provided. The slots 30 provide communication between the cavity 14 and a vacuum manifold 32.

The vacuum manifold 32 comprises a flanged annular L-cross section member 34 welded to each separable hemispherical portion of the metal sphere 12. The flanges 36 on annular members 34 are held together by bolts 38 (FIG. 7) which thereby serve to secure the two, separable hemispheres together three gold O-rings 40, 40a, and 40b of increasing diameters are positioned between the confronting flanges 36 to seal the interior of the metal sphere 12. An air outlet line 42 is connected to the space between the two innermost O-rings 40 and 40a, and leads to a mechanical pump (not shown) to reduce any leakage therethrough.

The main pumping system (FIG. 5) comprises a diffusion pump 44, connected through an appropriate oil vapor bafiie 46 and shut-off valve 48 to the vcuum manifold 32. Any standard diffusion pump such as an MCF-300 pump using octoil may be used in the pumping system which will operate to evacuate the interior of the metal sphere 12 at a pumping rate of approximately 50 liters per second.

The oil vapor bafile 46 comprises a tank 50 which is connected by means of line 52 to a liquid nitrogen source (not shown). A number of concentric copper baffies 54 surround the tank 50 and are interconnected by webs (not shown) to allow the passage of gases therethrough. An extra baffie plate 56 minimizes the effects of oil vapor back-streaming from the difiusion pump.

The shut-off valve 48 comprises a housing 57 within which is a disk 58 actuatable by a spindle 60 and a hand wheel 62 to lodge it against a valve seat 64. A flexible baffle 66 is connected between the disk 58 and the valve housing to prevent the passage of gas through the spindle mechanism. The shut-otf valve operates to isolate the vacuum system from the steel sphere during the formation and compression of the plasma discharge. It is to be understood that other types of vacuum pumps, such as the continuous ion-getter type, and shut-oil valves as well as baflie systems may be used to establish the vacuum within the sphere 12. Whatever pumping system is used, however, it should be capable of reducing the pressure within the sphere to approximately 10- millimeters of mercury.

A gas inlet line 68 connected to the manifold 32 supplies the gas to be ionized into the interior of the metal sphere 12. The mixture used in the embodiment herein described comprises approximately 50% by volume of each of the gases tritium and deuterium and it is introduced to a base pressure of approximately 10- to 10- millimeters mercury. A vacuum pressure gauge 70 con nected to the manifold 32 by a gas line 72 measures the pressure within the sphere. A pressure gauge of the Bayard-Alpert ion type may be used as well as any other pressure gauge which is capable of measuring high vacua.

To dissipate the heat in the cavity walls, cooling coils 74 surround the metal sphere 12. Water or other suitable cooling means is circulated through the cooling coils 74 by a pump 76 which passes the heated water to a conventional heat exchanger 78. The heat transferred in the heat exchanger may be dissipated to the atmosphere or Radius of plasma, In

it) it may hewonvertefd; tot}eleetrical-i pbwerxthrbugh well nuclear reactors'l-rriust :ber:accornpaniedirbyusubstaritiah known steam can I b'n gene tor means. reductionsz 'in 'kinnelfect power-dissipation Suchrreducm Referring to th'e' operatiari f the"devic-hereinbefore tion iwouldnbe:quitesefiectivemfor;instance,if? the; skin;

described,the cavity 14 withinzthe.. metal phe ilo sses could=.beireducedzby a;factor?of; lOO-LWhereimlthe; l 1365/ 'to i e desired pres r approx mately breakeeyenrpowenxgiven ;above;.:would1 bi'loweredrbyihei ifiereiiry'f'lhesyst'emis then preferably square oflithersanie gnumherzto he onceivable aluet outgassed by bakih'g 'the eiitife 'isysfern iiii a baking oven v t 1 at an elevatedtemperatur th ,.i hb hood QC. Qki tt hp system @The t q -e w r 1 eyiglent lthatthe:heatingtin the metalilwalls ise'to b regarded as a loss only if the power involvedgimustdiea E 2 LdeYiCSaIid lfdl baclcintohthema'g- W :poS Sible-afontheifieldvtwhegrnairih tainecl directly; by he ;fast;@charged;:reactions aproducts; th kin (ii atro would then represent lusef ul rpoweru ansferg frorn fasttchargedi particles does 7. 65 171116411; such electronicsdevicesins; may becar glledathafilany of the: electromagnetic -fi'e'ld;'- the ionized-partiole ducedawit'liihf thetvolunre'finte'rion 'to the maximumfield will abadirectedstowardihecenter ofit'hi'c'avitythnsi estab ilishing the plasma. The magnetic confining force of Q 1 JlCJJ approximately 420 gauss is established at the plasma inter- It will be seen that the Invention .as herelntfefore face and compresses the plasma to a volume approximatflescnbed fi the oblects for'th It ing 020 meter diameten intended to limit the present invention by the foregoing The principal design characteristics of the hereinbefore fiescnpilont but Instead, referenfi? 15 made to the w described device are Summarized below. 25. lifigeelltel ggs for the precise definition of the scope of this T able ll What is claimed is: Cavity diameter 5 1. A device for confining and compressing a plasma Empty cavity frequency, TE dlscharge comprising a metal housing defining a spherical mode 715 cavity, said housing having a plurality of equi-spaced Loaded cavity frequency 800 me. ports located on a great circle of said spherical cavity, Empty cavity Q (a figure of merit means for exciting electromagnetic waveswithm the said as Shown in Column 4) -10000Q cavity of the even and odd TE modes in time quadra- RF pulse length 140 milliseconds ture to rotate about-an axis of said spherical cavity per- RF pulse power pendrcular to the plane including said great circle, means Cavity fining time -25 microseconde 111 communication with said ports for introducing a heavy RF plate Supply capacitor 105 joules, 50 hydrogen isotope gas into the cavity at extremely low Cavity ultimate pressure 4 mm Hg pressure, whereby the gas Wlll be ionrzed and compressed Operating base pressure to mm. Hg towards the center of the spherical cavity by said electro- Floor space requirement 1500 40 Z I t IZ d i e of'claim 1 corn risin in addition means t is interesting To inquire if devices Such as herein for establishing a DC. magnetic fieldgparallel to seid axis before described can be scaled up to actual power ref rotation lctOrS-v The magnitudes 1 appear quite n- 3. A device for confinin and com ressin a lasrna able; however, it is an unpleasant fact that any contaet discharge comprising a metil sphere dgfininggaepgerieal ge the magnetic fiejld1 and ilr g g f ai g cavity, means for exciting electromagnetic waves within 6 11C 015 causes t 6 Power ance 0 e Omlna said cavit of the even and odd TE modes in time by skin losses in these conductors. The result is to raise quadrature, to rotate about an i di h center h minimum q y and POWer levels seemingly of said spherical cavity, and means for introducing a gas impractical regions several orders of magnitude higher comprising heavy hydrogen isotopes i the cavity at than that in t above eXafhP1efOUPWmg table extremely low pressure; whereby the gas will be ionized contains some c l fl P Y relatlonshlps f two and compressed towards the center of said cavity by said diiferent sized machines wherein the field shown In FI G. electromagnetic waves 7 3 0f the drawings is used to confine Plasma at a denslty 4. The device of claim 3 comprising in addition means of 2X1021 P Infiters and a temperature of 10 for providing a DC. magnetic field parallel to said axis Electrical constants for the walls are taken to be those f rotation of silver.

5. A method for confining and compressing a plasma discharge comprising evacuating a radio-frequency spherical resonant cavity, introducing a heavy hydrogen isotope r q n m p gas into said cavity at extremely low pressure, andex- Radlus 0 1 citing electromagnetic waves of the even and odd TE 00134 modes in time quadrature to rotate about an axis through 164, 000 a 5,200

324 0,324 the center of said spherical cavity, whereby sa1d gas will be ionized and the charged particles will be directed It is seen that the power situation is completely domitowards the center of Said cavity- Table 111 Power input to walls, mw. Power from T-D reaction, mw

nated bythe skin effect in the metal walls. The power method Confihlng and h -p a p f level at which the input to the wall is equal to the therdlscharge p s v p s a eq y ph r monuclear output from the plasma is independent of size l remhant Cal/1W, eXcItlIlg electrflmaghehc Waves lthln and depends only on the plasma temperature and the 531d l of even and 1n modes In q geometry of the system. For the particular temperature ture to rotate about an axis thro g t e Cen er of said and proportions used in calculating the values in the table, Spherlcal P W, and lnttroduclhg heavy y o this break-even power is approximately 0153x10 watts.- p gas into fy at eXtTemelYg 10W p fi Consideraton of the data in the preceding table towhereby the gas Will be ionized and the charged particles gether with the associated discussion shows that practical W ll be directed towards the center of the cavity.

use for power production of such machines as thermo- 7. A method for confining and compressing a plasma 

1. A DEVICE FOR CONFINING AND COMPRESSING A PLASMA DISCHARGE COMPRISING A METAL HOUSING DEFINING A SPHERICAL CAVITY, SAID HOUSING HAVING A PLURALITY OF EQUI-SPACED PORTS LOCATED ON A GREAT CIRCLE OF SAID SPHERICAL CAVITY MEANS FOR EXCITING ELECTROMAGNETIC WAVES WITHIN THE SAID CAVITY OF THE EVEN AND ODD TE111 MODES IN TIME QUADRATURE TO ROTATE ABOUT AN AXIS OF SAID SPHERICAL CAVITY PERPENDICULAR TO THE PLANE INCLUDING SAID GREAT CIRCLE, MEANS IN COMMUNICATION WITH SAID PORTS FOR INTRODUCING A HEAVY HYDROGEN ISOTOPE GAS INTO THE CAVITY AT EXTREMELY LOW PRESSURE, WHEREBY THE GAS WILL BE IONIZED AND COMPRESSED TOWARD THE CENTER OF THE SPHERICAL CAVITY BY SAID ELECTROMAGNETIC WAVES. 